This project is focused on the identification of physiologically critical functions of NF-kB transcription factors and their regulators in health and disease, as well as the molecular mechanisms underlying these functions. NF-kB is a family of related dimeric transcription factors that serve as primary intracellular mediators during innate and adaptive immune responses. In addition, and importantly, dysregulation of NF-kB plays a major role in inflammatory and autoimmune diseases as well as numerous tumors. It is thus imperative to understand the functions and mechanisms of action of NF-kB factors, as this will be required to devise appropriate strategies for therapeutic interventions aimed at curtailing dysregulated NF-kB. To identify physiologic roles we make use of mouse models engineered to lack components of the NF-kB transcription factor family or their regulators. These mouse models are used to identify NF-kB functions involved in the development of the immune system and they are subjected to challenge with pathogens and experimentally induced diseases in order to educe critical roles of NF-kB components in health and disease. Our work is focused on alternatively activated NF-kB factors and on Bcl-3. The alternative NF-kB activation pathway is initiated by a subset of TNF receptors. Bcl-3 is an atypical IkB family member which functions as nuclear regulator of NF-kB activity. Recently we discovered that alternatively activated NF-kB complexes and Bcl-3 co-operate in the development of medullary thymic epithelial cells, which in turn are required for proper negative selection (elimination) of self-reactive T cells. In FY 2010 we have obtained definitive proof that loss of the alternative pathway and Bcl-3 does indeed allow for the escape of self-reactive T cells into the periphery. In FY 2010 we have also generated the tools that will allow us to embark on a long-term effort to comprehensively investigate the roles of the alternative NF-kB pathway and of Bcl-3 in health and disease. We have generated mice in which the alternative pathway can be conditionally activated in various cell types. Previously we showed that BAFF-mediated alternative activation in B cells is essential for survival of B cells throughout peripheral development. In FY 2010 we have now determined that constitutive activation of this pathway in B cells leads to massive B cell hyperplasia, particularly in the gut. In FY 2010 we have also generated mice that are conditionally deficient in Bcl-3 and mice that are conditionally inducible to express Bcl-3. In FY 2010 we have identified a role for Bcl-3 in B cell differentiation. Both the alternative pathway and Bcl-3 have been implicated in B cell neoplasias in humans. The tools we have generated will allow us to dissect specific roles of the alternative pathway and Bcl-3 not only in neoplastic transformation of B cells, but also in B cell homeostasis, autoimmunity and thymic T cell selection.